When a qualitative analysis or a quantitative analysis on a sample is performed based on a peak in a chromatogram, a spectrum, or the like, the end points of the peak are first determined, and a quantitative analysis is performed using numerical values of the peak width, the peak intensity, the peak area, and the like of the peak defined by the determined peak end points. At this time, the measured peak waveform is approximated by (or fitted to) a known function form such as the form of a Gaussian function or a Lorentzian function to facilitate the determination of the peak end points, so that the numerical values of the peak width, the peak intensity, the peak area, and the like can be easily calculated.
However, a peak waveform in a chromatogram, a spectrum, or the like is not often obtained as a typical curve such as a known function form, and a baseline drift in which a baseline corresponding to background components rises or drops exists in some cases. Moreover, due to various noise components such as noise generated in a detector used for measurement, minor fluctuations arise on the baseline in some cases. This unsmooth waveform obtained through measurement is one of the causes for deteriorating the accuracy of a quantitative analysis.
The baseline of a peak is generally defined by a line connecting the start point (rising position) and the finish point (falling position) of the peak. Because the detection accuracy of the baseline of a peak directly influences the size of the peak area, inappropriate setting of the baseline, that is, inappropriate determination of the start point and the finish point of the peak, is another one of the causes for lowering the accuracy of a quantitative analysis.
Accordingly, in order to improve the accuracy of a quantitative analysis, it is required to appropriately determine the start point and the finish point of a peak even if a baseline drift exists.
FIG. 1 illustrates a method for determining the start point and the finish point of a peak while eliminating influences of a baseline drift. This method includes: obtaining a second order differential waveform 12 of a peak waveform 10; determining a threshold value 13 for a second order differential value from estimated noise intensity and the like; and determining a start point 14 and a finish point 15 of a peak in the peak waveform 10 from the relation between the second order differential waveform 12 and the threshold value 13 (see Non Patent Literature 1). Specifically, the points that intersect with the a line having the threshold value 13 at the foots of the second order differential waveform 12 outside the left local maximum point peak 17A and the right local maximum point peak 17B (which respectively correspond to the inflection points in the peak waveform 10) flanking the dip 16 are determined as the start point 14 and the finish point 15. The baseline 11 is the line connecting the start point 14 and the finish point 15.
FIG. 2 illustrates a plurality of peak waveforms respectively magnified (expanded/shrunk) at different vertical magnification ratios. When, as described above, a fixed threshold value 22 is applied to every peak to determine the start point and the finish point of a peak in each of the plurality of peak waveforms, the start point and the finish point are at A and A′ in the low magnification peak waveform 20, whereas they are at D and D′ in the high magnification peak waveform 21. This means that the start point and the finish point of the peak are different depending on the magnification ratio of each peak waveform. This leads to a problem that the linearity of the peak area among peaks magnified at different magnification ratios is not assured. The linearity is critical in creating a calibration curve in a quantitative analysis.
The peak waveforms in FIG. 2 are examples of asymmetric peak waveforms each having a tailing on one side. Due to the tailing, the distance between the finish points A′ and D′ is longer than the distance between the start points A and D between the low magnification peak waveform 20 and the high magnification peak waveform 21. In this way, in the case where a peak shape is asymmetric due to a tailing, a leading or the like, the detected start point and the finish point of a peak tend to be significantly shifted toward a peak top position, and this may adversely affect the peak area reproducibility. The “peak top position” refers to the point at which the peak intensity is highest. Here, “the point at which the peak intensity is highest” is not limited to a mathematically strict highest but includes a point at which the peak intensity is substantially highest.
According to such a conventional method in which a threshold value is uniformly applied to every peak, peak end points such as the start point and the finish point of a peak cannot be detected with high accuracy, and the accuracy of a quantitative analysis becomes lower in some cases.